Poling process for linear piezoelectric strain transducers

ABSTRACT

A process for producing sufficient piezoelectric effect in plastic dielectric coaxial cable to obtain a highly sensitive linear strain transducer. The process combines a selected soak temperature profile with the application of high voltage DC to maximize sensitivity and uniformity. The plastic dielectric material exhibits a high level of piezoelectric activity when processed according to the invention so that, for example, the cable is sensitive to the passage of humans or vehicles when buried in the ground.

ite States Patent [1 1 Ayers et a1.

[ POLING PROCESS FOR LINEAR PIEZOELECTRIC STRAIN TRANSDUCERS [75] Inventors: Weston D. Ayers, W. Covina; Joseph G. Hirsch, Diamond Bar, both of Calif.

[73] Assignee: General Dynamics Corporation,

Pomona, Calif.

22 Filed: Aug. 16,1973

211 Appl.No.:389,098

' [52] US. Cl 29/2535, 3l0/8.7, 340/17, 340/261 [51] Int. Cl B0lj 17/00 [58] Field of Search 29/2535; 340/10, 17,261; 174/110 PM; 252/629; 310/82, 8.3, 8.5,

[56] References Cited UNITED STATES PATENTS 2,900,536 8/1959 Palo 310/96 [4511 Jan. 28, 1975 3,750,127 7/1973 Ayers et al.... 340/261 3,820,208 6/1974 Baldy 29/2535 Primary Examiner-Carl E. Hall Attorney, Agent, or Firm-Neil F. Martin; Edward B. Johnson [57] ABSTRACT A process for producing sufficient piezoelectric effect in plastic dielectric coaxial cable to obtain a highly sensitive linear strain transducer. The process combines a selected soak temperature: profile with the application of high voltage DC to maximize sensitivity and uniformity. The plastic dielectric material exhibits a high level of piezoelectric activity when processed according to the invention so that, for example, the cable is sensitive to the passage of humans or vehicles when buried in the ground.

7 Claims, 5 Drawing Figures HIGH VOLTAGE SOURCE 1 POLING PROCESS FOR LINEAR PIEZOELECTRIC STRAIN TRANSDUCERS BACKGROUND OF THE INVENTION Various experiments have been conducted into the piezoelectric characteristics of plastic materials and it has been suggested that the piezoelectric characteristic of plastic materials may be enhanced in some circumstances by a poling process. The voltages associated with the known prior art poling processes have been on the order of 30,000 volts DC and therefore are difficult to apply in situations where conductors are in proximity to other conductive elements, or where the dielectric strength of the insulative materials is insufficient to avoid breakdown.

Other prior art studies have explored the characteristics of coaxial cable wherein flexing of the cable produces noise by virtue of the generation of a charge in accordance with what has been referred to as a tribeelectric effect. This effect is generally assumed to result from friction between the dielectric and the conductor and shield. However, the potential of coaxial cable as a sensing element was not fully realized until the development of applicant's Method and Means for Sensing Strain with a Piezoelectric Strain Sensing Element," Ser. No. 193,419, filed Oct. 28, 1971, now U.S. Pat. No. 3,750,127, the subject matter of which prior patent is hereby incorporated herein by reference. In accordance with the principles of the invention as recited in U.S. Pat. No. 3,750,127, certain types of coaxial cable exhibit sufficient piezoelectric effect generated by the plastic dielectric to make it useful as a strain transducer in applications where the level of strain and modulus of the surrounding medium are quite small. The techniques suggested by the prior art poling processes have not been applicable to an already manufactured coaxial cable due especially to the excessive voltages involved, and the inability to obtain consistent and uniform results.

It is therefore desirable to have a poling process for coaxial cable that consistently develops a uniform high sensitivity piezoelectric strain transducer while maintaining the other physical and electrical properties of manufactured coaxial cable.

SUMMARY OF THE INVENTION In the exemplary embodiment, coaxial cable with a plastic dielectric is wound with minimum tension on a large diameter cable spool, preferrably in even, closely spaced layers. The barrel of the spool is preferrably padded to absorb any stress which would otherwise be transferred to the lower layers during the heating and cooling cycle.

The outer end of the cable is laid back to produce an air gap between the center conductor and shield that will prevent arcing over during the application of high voltage DC. The cable is placed in a temperature chamher and a source of high voltage DC connected between the shield and center conductor. The temperature chamber is then raised to a temperature above the transition temperature for the plastic dielectric. At this temperature, and under the influence of the applied high voltage DC, the piezoelectric generating regions move into alignment with the electric field. The process continues by holding the chamber at the elevated temperature for a selected period of time, after which a cooling process begins with a selected initial cooling rate. After cooling for a selected period of time at the initial cooling rate, the final cooling process may commence at another selected rate. The resulting poled cable has a high initial piezoelectric sensitivity which decays exponentially. Therefore, in order to obtain consistent operating characteristics, the cable is aged to overcome the high initial decay rate.

It is therefore an object of the invention to provide a new and improved poling process for linear piezoelectric strain transducers.

It is another object of the invention to provide a new and improved poling process for use in obtaining piezoelectric characteristics for the dielectric in coaxial cable.

It is another object of the invention to provide a new and improved poling process that provides a high piezoelectric sensitivity with a relatively low voltage bias during a heating cycle.

It is another object of the invention to provide a new and improved poling process with maximum retention of the piezoelectric characteristics over the transducers lifetime.

It is another object of the invention to provide a new and improved polingprocess that may be carried out on coaxial cable wound on a drum.

It is another object of the invention to provide a new and improved poling process which minimizes the stress on the cable during heating and voltage cycle.

It is another object of the invention to provide a new and improved poling process that is especially applicable to coaxial cable with polyethylene dielectric.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a graph of the heat treatment cycle used in the process.

FIG. 2 is a perspective view of a typical spool with a length of cable wound thereon for treatment.

FIG. 3 is an enlarged sectional view taken on line 3-3 of FIG. 2.

FIG. 4 is a perspective view of the prepared end of the cable.

FIG. 5 illustrates the treatment apparatus as used.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the drawings, there is illustrated a coaxial cable 10. In the preferred embodiment, the cable comprises RG 58 c/u and has substantially a one quarter inch outside diameter. The shield 12 is spaced from the center conductor 14 by polyethylene dielectric 16. While it is possible to utilize coaxial cable incorporating other dielectrics such as tetrafluoroethylene, polyethylene is preferred because of its relatively low transition temperature and chemical resistance. The cable 10 is uncoiled from the shipping drum and is preferrably wound on a special poling drum 20 as illustrated in FIGS. 2 and 3. Drum 20 has a relatively large diameter barrel portion 22 so that substantial lengths of cable 10 can be wound onto the barrel 22 without there being an excess of layers thereon. In practice it has been found that depths of four layers or less of RG 58 c/u cable can be efficiently and suitably processed, in accordance with the present invention. Too many layers create stresses which cause distortions and result in non-uniform cable sensitivity. The barrel portion 22 is preferrably padded with a cotton padding 3 layer 24, for example, that acts to absorb any thermally induced stresses or other stresses which would otherwise be transferred to the cable axially or transversely.

ln winding the cable 10 onto the drum 20, care is utilized to obtain even, closely spaced layers with minimum tension. After winding on the drum 20, the outer cable end 40 is dressed according to the illustration in FIG. 4. The spacing between the inner exposed end of conductor 14 and the start of the shield 12 is maintained at a minimum of 1 inch. This spacing insures that there will be no arcing across the air gap upon the application of the high voltage. A high voltage lead 30 is connected via screw connector 32 to the center conductor, and a second high voltage lead 34 is connected by a ring connector 36 to the shield 12. The leads 34 and 36 are connected to a high voltage cable 38 which is wired to a high voltage source 42. The voltage source 42 is capable of delivering at least 10,000 volts DC and is provided with an ammeter 44 as in FIG. 5. The drum 20 is inserted into the temperature chamber 46 and the door 48 closed to seal off the chamber. Then, the high voltage supply 42 is activated and the current reading on ammeter 44 is noted. If the ammeter reading is below 20 microamps, indicating an absence of any arcing conditions, the heating cycle may be commenced.

Referring to the graph in FIG. 1, the time temperature profile is illustrated. The chamber is brought up to the maximum and holding temperature of substantially 200 F within a period of 1 hour as is indicated by the segment 60. Horizontal segment 62 indicates that the chamber is held at the soak temperature of substantially 200 F for a period of 7 hours. The cooling cycle commences with segment 64 and is at an initial rate of F per hour. The low initial cooling rate insures against the loss of polarization, and therefore sensitivity, from thermally induced stresses.

The final segment 66 continues the cooling cycle but at a rate of F per hour resulting in completion of the cooling cycle at the end of 24 hours from commencement.

For most uses, it is important that a relatively constant piezoelectric sensitivity be maintained, and therefore it is necessary to age the activated cable. Aging at room temperature for a period of 2 to 4 weeks results in a decay of sensitivity over the highly sloped portion of an exponential curve to the point where sensitivity is substantially stable over useful periods of time. It is possible to accelerate the aging process by raising the cable to a temperature below the soak temperature and thereby shorten the aging cycle. Such processes are considered to be equivalent to the room temperature aging approach.

After aging, the coaxial cable is useful as a transducer and acts as a linear strain gauge. The linear nature of the strain gauge and of the piezoelectric transducing element result in an integration of the strain, and therefore produces a high signal output for relatively small transmitted forces. ln normal use, the center conductor and shield, which form the two electrodes of the strain gauge, are connected to a charge follower producing a highly sensitive detection of transmitted stress.

It should be understood that although a specific cable type, temperatures, voltage and rate cycles, etc., have been specified herein above in describing the method of the invention that the values for those items would not necessarily be the same for processing different cables. In addition, the cables to be processed may be wound in the form of flat spirals or rings, for example. It can easily be seen that short lengths of cable would not require coiling or winding, if the heat chamber were of sufficient size to hold the cable lengths desired.

We claim:

1. A method of poling manufactured coaxial cable having a plastic dielectric material disposed intermediate the inner conductor and the outer shield conductor of the cable to produce a linear piezoelectric transducer comprising the steps of:

padding the barrel portion of a poling drum;

loosely winding a length of the manufactured coaxial cable around said padded barrel portion of said drum; dressing the outer end of said cable to obtain a minimum of a 1 inch air gap between the inner conductor and the outer shield conductor of said cable;

placing said drum holding said cable in a temperature chamber;

connecting a source of high voltage to the dressed end of said cable between the inner conductor and the outer shield conductor of said cable;

heating said chamber and said cable to an elevated temperature;

cooling said chamber and said cable;

disconnecting said source of high voltage; and aging said cable to overcome its high initial piezoelectric sensitivity which decays exponentially.

2. The method of claim 1 wherein:

said source of high voltage has a potential of substantially 10,000 volts DC.

3. The method of claim 1 wherein:

said cable has a polyethylene dielectric, and

said chamber is heated to a temperature in excess of 4. The method of claim 3 wherein:

said chamber is heated to substantially 200 F.

5. The method of claim 3 wherein:

said chamber is held at said elevated temperature for substantially 7 hours.

6. The method of claim 5 wherein:

said chamber is cooled to room temperature over a period of substantially 16 hours.

7. The method of claim 5 wherein:

said chamber is cooled at an initial rate not in excess of 5 F per hour. 

1. A method of poling manufactured coaxial cable having a plastic dielectric material disposed intermediate the inner conductor and the outer shield conductor of the cable to produce a linear piezoelectric transducer comprising the steps of: padding the barrel portion of a poling drum; loosely winding a length of the manufactured coaxial cable around said padded barrel portion of said drum; dressing the outer end of said cable to obtain a minimum of a 1 inch air gap between tHe inner conductor and the outer shield conductor of said cable; placing said drum holding said cable in a temperature chamber; connecting a source of high voltage to the dressed end of said cable between the inner conductor and the outer shield conductor of said cable; heating said chamber and said cable to an elevated temperature; cooling said chamber and said cable; disconnecting said source of high voltage; and aging said cable to overcome its high initial piezoelectric sensitivity which decays exponentially.
 2. The method of claim 1 wherein: said source of high voltage has a potential of substantially 10, 000 volts DC.
 3. The method of claim 1 wherein: said cable has a polyethylene dielectric, and said chamber is heated to a temperature in excess of 180* F.
 4. The method of claim 3 wherein: said chamber is heated to substantially 200* F.
 5. The method of claim 3 wherein: said chamber is held at said elevated temperature for substantially 7 hours.
 6. The method of claim 5 wherein: said chamber is cooled to room temperature over a period of substantially 16 hours.
 7. The method of claim 5 wherein: said chamber is cooled at an initial rate not in excess of 5* F per hour. 